1. Field of the Invention
The present invention relates to an image reading apparatus and, more particularly, to removal of flare in an image reading apparatus using a slit exposure optical system.
2. Related Background Art
FIG. 1 is a schematic view showing the main part of a conventional image reading apparatus used in a digital copying machine, flat bed scanner, or the like.
Referring to FIG. 1, an original 118 placed on an original glass table 112 is illuminated from the two sides with a direct light beam from a light source 171 and a light beam incident through a reflector 172. The light beam reflected by the original 118 passes through a stop (slit) 174 and is focused on the surface of a CCD (line sensor) 113 by an imaging lens 111 through first, second, and third scanning mirrors 114, 115, and 116. The optical image formed on the surface of the CCD 113 is converted into an electrical signal in accordance with the density of the original 118, and one-line image information (in the main scanning direction (direction perpendicular to the drawing sheet of FIG. 1) is read. Image information in the sub-scanning direction (direction indicated by an arrow Y in FIG. 1) is read as follows. The first mirror table comprised of the light source 171, a reflector 172, the stop 174, and the first scanning mirror (full-speed scanning mirror) 114 is moved in the sub-scanning direction with respect to the original 118. The second mirror table comprised of the second and third scanning mirrors (half-speed mirrors) 115 and 116 is moved in the sub-scanning direction at a speed half (moving speed ratio of 2:1) that of the first mirror table. The image information of the original 118 is read while the optical path length from the original 118 to the CCD 113 is kept constant.
The light source, reflector, slit, first to third scanning mirrors, and CCD in the image reading apparatus of this type stretch in a direction (main scanning direction) perpendicular to the page of drawing to read the entire line of the original in the main scanning direction and the entire area of the original by scanning the first and second mirror tables. This optical system is a so-called slit exposure optical system.
The CCD pixel array is composed of one line for a monochrome image read and three adjacent lines (monolithic 3-line sensor) of R, G, and B for a color image read. The magnification of the imaging lens is determined by the original read pitch and CCD pixel pitch. For example, when the read pitch is 600 DPI and the CCD pixel pitch is 7 xcexcm, the imaging magnification xcex2 is xc3x970.16535.
The above image reading apparatus suffers from the following various problems. Such a problem is described with reference to FIG. 2. FIG. 2 is a schematic view showing the main part of the optical system of the image reading apparatus in FIG. 1 from the viewpoint of paraxial optics. For the descriptive convenience, as the main constituent elements in FIG. 1, only the imaging lens 111, original glass table 112, and CCD 113 are extracted. At the same time, an optical axis O of the imaging lens 111, marginal light rays 10, and typical light rays 14, 15, and 16 are illustrated.
Along with the higher integration of recent CCD substrates, the semiconductor process techniques have been re-examined to increase the yield, and the surface precision of electrodes on the CCD substrate has improved. As the CCD surface has become an almost mirror surface, the imaging light beam incident on or near the CCD pixel array is partially reflected by the CCD surface. For example, the light ray 14 (travelling in the A direction) as part of the imaging light beam is partially reflected by a pixel array 131 of the CCD 113 and becomes a reflected light ray (flare ray) 15 (travelling in the B direction). Most of the reflected light ray 15 is refocused near a surface 121 of the original glass table 112 by the imaging lens 111. The light ray 15 reflected by the surface 121 of the original glass table 112 travels as the light ray (propagating in the C direction) tracing nearly the same optical path as the light ray 14, and reaches the surface of the CCD 113.
As described above, a flare due to multiple reflection between the original glass table 112 and the CCD 113 tends to be produced on or near the axis, but is hardly produced at an off-axis position due to the following reason. That is, as an off-axis light beam has a larger incident angle on the CCD 113 due to increased field angle, the light ray 15 reflected by the surface of the CCD 113 can hardly enter the imaging lens 111 again.
For example, when an image having a uniform intermediate density is read, streaks of a white flare appear near the central portion of the resultant image. This interferes with high image quality.
It is an object of the present invention to provide a low-cost, compact image reading apparatus in which a reading means is not perpendicular to the optical axis of an imaging optical system, or the optical axis between an original glass table and a first scanning mirror is not perpendicular to the original glass table to eliminate the multiple reflection flare between the original surface and reading means surface and improve the image quality of the image reading apparatus.
According to the present invention, an image reading apparatus for causing illumination means to illuminate an original placed on an original glass table, causing an imaging optical system to focus a light beam based on image information of the original surface on a surface of the reading means, and causing the reading means to read the image information is characterized in that the reading means is not perpendicular to an optical axis of the imaging optical system, and the following condition is satisfied:
xcex8ixe2x89xa7sinxe2x88x921(NAi)
where xcex8i is the tilt angle of the reading means with respect to the optical axis of the imaging optical system, and NAi is the numerical aperture of the imaging optical system on a reading means side. In particular, the following condition is satisfied:
xcex80xe2x89xa7sinxe2x88x921(NA0)
where xcex80 is the tilt angle of the original glass table with respect to the optical axis of the imaging optical system when the original glass table is not perpendicular to the optical axis of the imaging optical system, and NA0 is the numerical aperture of the imaging optical system on an original side.
The image reading apparatus is characterized in that a light-shielding member is inserted between the imaging optical system and the reading means, and a normal to the reading means is directed toward the light-shielding member. Alternatively, the image reading apparatus is characterized in that a plurality of scanning mirrors are inserted between the original glass table and the imaging optical system.
According to the present invention, an image reading apparatus for causing illumination means to illuminate an original placed on an original glass table, causing an imaging optical system to focus a light beam based on image information of an illuminated original surface on a surface of the reading means through a plurality of scanning mirrors, and causing the reading means to read the image information is characterized in that an optical axis between the original glass table and a first scanning mirror of the plurality of scanning mirrors which opposes the original glass table is not perpendicular to the original glass table, and the following condition is satisfied:
xcex80xe2x89xa7sinxe2x88x921(NA0)
where xcex80 is the angle a normal to the original glass table makes with the optical axis between the original glass table and the first scanning mirror, and NA0 is the numerical aperture of the imaging optical system on an original side.
According to the present invention, an image reading apparatus for moving an integral scanning optical system unit in a scanning direction to read image information of an original, the integral scanning optical system unit integrally housing illumination means for illuminating the original placed on an original glass table, a plurality of scanning mirrors for reflecting a light beam from the original illuminated with the illumination means, an imaging optical system for focusing a light beam reflected by the plurality of scanning mirrors, and reading means arranged at an imaging position of the imaging optical system, is characterized in that an optical axis between the original glass table and a first scanning mirror of the plurality of scanning mirrors which opposes the original glass table is not perpendicular to the original glass table, and the following condition is satisfied:
xcex80xe2x89xa7sinxe2x88x921(NA0)
where xcex80 is the angle a normal to the original glass table makes with the optical axis between the original glass table and the first scanning mirror, and NA0 is the numerical aperture of the imaging optical system on an original side.
According to the present invention, an image reading apparatus for moving an integral scanning optical system unit in a scanning direction to read image information of an original, the integral scanning optical system unit integrally housing illumination means for illuminating the original placed on an original glass table, an imaging optical system for focusing a light beam from the original illuminated with the illumination means, and reading means arranged at an imaging position of the imaging optical system, is characterized in that the reading means is not perpendicular to an optical axis of the imaging optical system, and the following condition is satisfied:
xcex8ixe2x89xa7sinxe2x88x921(NAi)
where xcex8i is the tilt angle of the reading means with respect to the optical axis of the imaging optical system, and NAi is the numerical aperture of the imaging optical system on a reading means side. In particular, the following condition is satisfied:
xcex80xe2x89xa7sinxe2x88x921(NA0)
where xcex80 is the tilt angle of the original glass table with respect to the optical axis of the imaging optical system when the original glass table is not perpendicular to the optical axis of the imaging optical system, and NA0 is the numerical aperture of the imaging optical system on an original side.
The image reading apparatus is characterized in that a light-shielding member is inserted between the imaging optical system and the reading means, and a normal to the reading means is directed toward the light-shielding member. Alternatively, the image reading apparatus is characterized in that a plurality of scanning mirrors are inserted between the original glass table and the imaging optical system.
According to the present invention, an image reading apparatus for moving an integral scanning optical system unit in a scanning direction to read image information of an original, the integral scanning optical system unit integrally housing illumination means for illuminating the original placed on an original glass table, a plurality of scanning mirrors for reflecting a light beam from the original illuminated with the illumination means, an imaging optical system for focusing a light beam reflected by the plurality of scanning mirrors, and reading means arranged at an imaging position of the imaging optical system, is characterized in that an optical axis between the original glass table and a first scanning mirror of the plurality of scanning mirrors which opposes the original glass table is not perpendicular to the original glass table, and the following condition is satisfied:
xcex80xe2x89xa7sinxe2x88x921(NA0 )
where xcex80 is the angle a normal to the original glass table makes with the optical axis between the original glass table and the first scanning mirror, and NA0 is the numerical aperture of the imaging optical system on an original side.
According to the present invention, an image reading apparatus for moving an integral scanning optical system unit in a scanning direction to read image information of an original, the integral scanning optical system unit integrally housing illumination means for illuminating the original placed on an original glass table, an imaging optical system for focusing a light beam from the original illuminated with the illumination means, and reading means arranged at an imaging position of the imaging optical system, is characterized in that the reading means is not perpendicular to an optical axis of the imaging optical system, and the following condition is satisfied:
xcex8ixe2x89xa7sinxe2x88x921(NAi)
where xcex8i is the tilt angle of the reading means with respect to the optical axis of the imaging optical system, and NAi is the numerical aperture of the imaging optical system on a reading means side. In particular, the following condition is satisfied:
xcex80xe2x89xa7sinxe2x88x921(NA0)
where xcex80 is the tilt angle of the original glass table with respect to the optical axis of the imaging optical system when the original glass table is not perpendicular to the optical axis of the imaging optical system, and NA0 is the numerical aperture of the imaging optical system on an original side.
The image reading apparatus is characterized in that a light-shielding member is inserted between the imaging optical system and the reading means, and a normal to the reading means is directed toward the light-shielding member. Alternatively, the image reading apparatus is characterized in that a plurality of scanning mirrors are inserted between the original glass table and the imaging optical system.